Gamer has been removed
You will receive this achievement after completing Chapter 30: To The Lair.
You can choose to use the Level Select cheat which will allow you to select this specific chapter or you can simply play through the story mode and unlock the achievement when you come to it.
I would recommend playing through the entire story mode without using the level select cheat if you are intending on going for the full 1000G as the Completed Game achievement requires you to complete every chapter, not just the ones with achievements attached.
By pressing start, then entering the following you will unlock a cheat option:
Hold LB+LT+RB+RT - Down, Up, Y, X, Down, Down, Y, Y
Level Select: KKst0ry
Thanks to robinhoodfreak7 for creating video walkthroughs for each individual chapter, I have included it in the solution if you are encountering any difficulties: